A concern regarding radio frequency (RF) leakage current is present in any electrosurgical unit (ESU). RF leakage current refers to the small, but nevertheless sometimes significant, current which flows into the surrounding environment from the active electrode and the conductor which supplies the active electrode, when the surgeon has activated or "keyed" the ESU prior to bringing the active electrode into operative arcing distance from the tissue of the patient. There is a concern that the RF leakage current will flow to the surgeon and to those in the operating room, exposing the surgeon and others to risk of injury. Based on these concerns, and on safety regulations, the maximum allowable amount of RF leakage current which can flow from an ESU must be controlled and limited.
The RF leakage current is at its maximum during open-circuit, full-power operating conditions. When the ESU is keyed, but no arcs travel from the active electrode to the tissue, relatively high peak-to-peak voltages of full power cause the RF leakage current to more readily disperse into the surroundings. As soon as the active electrode is brought into operative distance from the tissue, and arcs are conducted to the tissue, the circuit is closed, the output voltage drops under this "loaded" condition, and the RF leakage current is no longer of a major concern because most or all of the power is delivered to the tissue. As soon as the conductive pathways are established to the tissue, RD leakage current is minimized due to the considerably lower impedance path of the ionized pathways in the gas jet to the tissue. The same concern with RF leakage current also occurs after the active electrode is pulled away an inoperative distance from the tissue, but the ESU remains keyed.
Beam-type ESUs have special power requirements which other types of ESUs do not have. A beam-type ESU is one which delivers electrical energy, usually arcs, in ionized conductive pathways established in a continuously flowing jet of a predetermined gas. U.S. Pat. No. 4,781,175 (Ser. No. 849,950) discloses a beam-type ESU. In a beam-type ESU, the gas flowing past the active electrode must be maintained in an ionized state. The ionized state allows the arcs to be reliably initiated from the active electrode through the gas jet to the tissue, when the pencil-like device which delivers the gas jet and contains the active electrode is brought into an operative distance with the tissue. Without maintaining a state of sufficient ionization, arcs will not initiate when the surgeon desires, or the initiation will not be as reliable and predictable as is desired. Maintaining the ionization state in beam-type ESUs can be difficult, because the continuous flow of gas past the electrode requires electrical energy to be continually delivered in substantial magnitudes to prevent the ionized state from extinguishing.
In a conventional ESU, gas is not constantly flowing past the active electrode. Furthermore, many conventional ESUs require actual physical contact or near physical contact of the active electrode with the tissue in order to initiate the arcs. Physical contact of the active electrode to the tissue is not desirable or possible in beam-type ESUs. Therefore, the constant state of ionization in the gas jet flowing from the active electrode must not only be maintained, but it must be maintained to a degree which allows the predictable initiation of arcs in the conductive pathways established by the ionization, once the active electrode is brought into operative proximity with the tissue.
It has been determined that an effective technique of maintaining an ionized state of ionized conductive pathways in a gas is to apply relatively high peak-to-peak voltage to the gas. However, maintaining the ionization state in the gas jet of a beam-type ESU by applying a relatively high peak-to-peak voltage has the detrimental effect of increasing the RF leakage current. Thus, the requirement to maintain an effective ionized state in the gas jet sufficient to reliably initiate arcs to the tissue when desired, and the requirement to limit the amount of RF leakage current, are both significant but contradictory considerations in beam-type ESUs.